Connecting to uncommon HID devices

The WebHID API allows websites to access alternative auxiliary keyboards and exotic gamepads.

François Beaufort

There is a long tail of human interface devices (HIDs), such as alternative keyboards or exotic gamepads, that are too new, too old, or too uncommon to be accessible by systems' device drivers. The WebHID API solves this by providing a way to implement device-specific logic in JavaScript.

Suggested use cases

An HID device takes input from or provides output to humans. Examples of devices include keyboards, pointing devices (mice, touchscreens, etc.), and gamepads. The HID protocol makes it possible to access these devices on desktop computers using operating system drivers. The web platform supports HID devices by relying on these drivers.

The inability to access uncommon HID devices is particularly painful when it comes to alternative auxiliary keyboards (e.g. Elgato Stream Deck, Jabra headsets, X-keys) and exotic gamepad support. Gamepads designed for desktop often use HID for gamepad inputs (buttons, joysticks, triggers) and outputs (LEDs, rumble). Unfortunately, gamepad inputs and outputs are not well standardized and web browsers often require custom logic for specific devices. This is unsustainable and results in poor support for the long tail of older and uncommon devices. It also causes the browser to depend on quirks in the behavior of specific devices.

Terminology

HID consists of two fundamental concepts: reports and report descriptors. Reports are the data that is exchanged between a device and a software client. The report descriptor describes the format and meaning of data that the device supports.

An HID (Human Interface Device) is a type of device that takes input from or provides output to humans. It also refers to the HID protocol, a standard for bi-directional communication between a host and a device that is designed to simplify the installation procedure. The HID protocol was originally developed for USB devices, but has since been implemented over many other protocols, including Bluetooth.

Applications and HID devices exchange binary data through three report types:

A report descriptor describes the binary format of reports supported by the device. Its structure is hierarchical and can group reports together as distinct collections within the top-level collection. The format of the descriptor is defined by the HID specification.

An HID usage is a numeric value referring to a standardized input or output. Usage values allow a device to describe the intended use of the device and the purpose of each field in its reports. For example, one is defined for the left button of a mouse. Usages are also organized into usage pages, which provide an indication of the high-level category of the device or report.

Using the WebHID API

Feature detection

To check if the WebHID API is supported, use:

if ("hid" in navigator) {
  // The WebHID API is supported.
}

Open an HID connection

The WebHID API is asynchronous by design to prevent the website UI from blocking when awaiting input. This is important because HID data can be received at any time, requiring a way to listen to it.

To open an HID connection, first access a HIDDevice object. For this, you can either prompt the user to select a device by calling navigator.hid.requestDevice(), or pick one from navigator.hid.getDevices() which returns a list of devices the website has been granted access to previously.

The navigator.hid.requestDevice() function takes a mandatory object that defines filters. Those are used to match any device connected with a USB vendor identifier (vendorId), a USB product identifier (productId), a usage page value (usagePage), and a usage value (usage). You can get those from the USB ID Repository and the HID usage tables document.

The multiple HIDDevice objects returned by this function represent multiple HID interfaces on the same physical device.

// Filter on devices with the Nintendo Switch Joy-Con USB Vendor/Product IDs.
const filters = [
  {
    vendorId: 0x057e, // Nintendo Co., Ltd
    productId: 0x2006 // Joy-Con Left
  },
  {
    vendorId: 0x057e, // Nintendo Co., Ltd
    productId: 0x2007 // Joy-Con Right
  }
];

// Prompt user to select a Joy-Con device.
const [device] = await navigator.hid.requestDevice({ filters });

// Get all devices the user has previously granted the website access to.
const devices = await navigator.hid.getDevices();

You can also use the optional exclusionFilters key in navigator.hid.requestDevice() to exclude some devices from the browser picker that are known to be malfunctioning for instance.

// Request access to a device with vendor ID 0xABCD. The device must also have
// a collection with usage page Consumer (0x000C) and usage ID Consumer
// Control (0x0001). The device with product ID 0x1234 is malfunctioning.
const [device] = await navigator.hid.requestDevice({
  filters: [{ vendorId: 0xabcd, usagePage: 0x000c, usage: 0x0001 }],
  exclusionFilters: [{ vendorId: 0xabcd, productId: 0x1234 }],
});

A HIDDevice object contains USB vendor and product identifiers for device identification. Its collections attribute is initialized with a hierarchical description of the device's report formats.

for (let collection of device.collections) {
  // An HID collection includes usage, usage page, reports, and subcollections.
  console.log(`Usage: ${collection.usage}`);
  console.log(`Usage page: ${collection.usagePage}`);

  for (let inputReport of collection.inputReports) {
    console.log(`Input report: ${inputReport.reportId}`);
    // Loop through inputReport.items
  }

  for (let outputReport of collection.outputReports) {
    console.log(`Output report: ${outputReport.reportId}`);
    // Loop through outputReport.items
  }

  for (let featureReport of collection.featureReports) {
    console.log(`Feature report: ${featureReport.reportId}`);
    // Loop through featureReport.items
  }

  // Loop through subcollections with collection.children
}

The HIDDevice devices are by default returned in a "closed" state and must be opened by calling open() before data can be sent or received.

// Wait for the HID connection to open before sending/receiving data.
await device.open();

Receive input reports

Once the HID connection has been established, you can handle incoming input reports by listening to the "inputreport" events from the device. Those events contain the HID data as a DataView object (data), the HID device it belongs to (device), and the 8-bit report ID associated with the input report (reportId).

Continuing with the previous example, the code below shows you how to detect which button the user has pressed on a Joy-Con Right device so that you can hopefully try it at home.

device.addEventListener("inputreport", event => {
  const { data, device, reportId } = event;

  // Handle only the Joy-Con Right device and a specific report ID.
  if (device.productId !== 0x2007 && reportId !== 0x3f) return;

  const value = data.getUint8(0);
  if (value === 0) return;

  const someButtons = { 1: "A", 2: "X", 4: "B", 8: "Y" };
  console.log(`User pressed button ${someButtons[value]}.`);
});

Send output reports

To send an output report to an HID device, pass the 8-bit report ID associated with the output report (reportId) and bytes as a BufferSource (data) to device.sendReport(). The returned promise resolves once the report has been sent. If the HID device does not use report IDs, set reportId to 0.

The example below applies to a Joy-Con device and shows you how to make it rumble with output reports.

// First, send a command to enable vibration.
// Magical bytes come from https://github.com/mzyy94/joycon-toolweb
const enableVibrationData = [1, 0, 1, 64, 64, 0, 1, 64, 64, 0x48, 0x01];
await device.sendReport(0x01, new Uint8Array(enableVibrationData));

// Then, send a command to make the Joy-Con device rumble.
// Actual bytes are available in the sample below.
const rumbleData = [ /* ... */ ];
await device.sendReport(0x10, new Uint8Array(rumbleData));

Send and receive feature reports

Feature reports are the only type of HID data reports that can travel in both directions. They allow HID devices and applications to exchange non standardized HID data. Unlike input and output reports, feature reports are not received or sent by the application on a regular basis.

To send a feature report to an HID device, pass the 8-bit report ID associated with the feature report (reportId) and bytes as a BufferSource (data) to device.sendFeatureReport(). The returned promise resolves once the report has been sent. If the HID device does not use report IDs, set reportId to 0.

The example below illustrates the use of feature reports by showing you how to request an Apple keyboard backlight device, open it, and make it blink.

const waitFor = duration => new Promise(r => setTimeout(r, duration));

// Prompt user to select an Apple Keyboard Backlight device.
const [device] = await navigator.hid.requestDevice({
  filters: [{ vendorId: 0x05ac, usage: 0x0f, usagePage: 0xff00 }]
});

// Wait for the HID connection to open.
await device.open();

// Blink!
const reportId = 1;
for (let i = 0; i < 10; i++) {
  // Turn off
  await device.sendFeatureReport(reportId, Uint32Array.from([0, 0]));
  await waitFor(100);
  // Turn on
  await device.sendFeatureReport(reportId, Uint32Array.from([512, 0]));
  await waitFor(100);
}

To receive a feature report from an HID device, pass the 8-bit report ID associated with the feature report (reportId) to device.receiveFeatureReport(). The returned promise resolves with a DataView object that contains the contents of the feature report. If the HID device does not use report IDs, set reportId to 0.

// Request feature report.
const dataView = await device.receiveFeatureReport(/* reportId= */ 1);

// Read feature report contents with dataView.getInt8(), getUint8(), etc...

Listen to connection and disconnection

When the website has been granted permission to access an HID device, it can actively receive connection and disconnection events by listening to "connect" and "disconnect" events.

navigator.hid.addEventListener("connect", event => {
  // Automatically open event.device or warn user a device is available.
});

navigator.hid.addEventListener("disconnect", event => {
  // Remove |event.device| from the UI.
});

Revoke access to an HID device

The website can clean up permissions to access an HID device it is no longer interested in retaining by calling forget() on the HIDDevice instance. For example, for an educational web application used on a shared computer with many devices, a large number of accumulated user-generated permissions creates a poor user experience.

Calling forget() on a single HIDDevice instance will revoke access to all the HID interfaces on the same physical device.

// Voluntarily revoke access to this HID device.
await device.forget();

As forget() is available in Chrome 100 or later, check if this feature is supported with the following:

if ("hid" in navigator && "forget" in HIDDevice.prototype) {
  // forget() is supported.
}

Dev Tips

Debugging HID in Chrome is easy with the internal page, about://device-log where you can see all HID and USB device related events in one single place.

Check out the HID explorer for dumping HID device info into a human-readable format. It maps from usage values to names for each HID usage.

On most Linux systems, HID devices are mapped with read-only permissions by default. To allow Chrome to open an HID device, you will need to add a new udev rule. Create a file at /etc/udev/rules.d/50-yourdevicename.rules with the following content:

KERNEL=="hidraw*", ATTRS{idVendor}=="[yourdevicevendor]", MODE="0664", GROUP="plugdev"

In the line above, [yourdevicevendor] is 057e if your device is a Nintendo Switch Joy-Con for instance. ATTRS{idProduct} can also be added for a more specific rule. Make sure your user is a member of the plugdev group. Then, just reconnect your device.

Browser support

The WebHID API is available on all desktop platforms (ChromeOS, Linux, macOS, and Windows) in Chrome 89.

Demos

Some WebHID demos are listed at web.dev/hid-examples. Go have a look!

Security and privacy

The spec authors have designed and implemented the WebHID API using the core principles defined in Controlling Access to Powerful Web Platform Features, including user control, transparency, and ergonomics. The ability to use this API is primarily gated by a permission model that grants access to only a single HID device at a time. In response to a user prompt, the user must take active steps to select a particular HID device.

To understand the security tradeoffs, check out the Security and Privacy Considerations section of the WebHID spec.

On top of this, Chrome inspects the usage of each top-level collection and if a top-level collection has a protected usage (e.g. generic keyboard, mouse), then a website won't be able to send and receive any reports defined in that collection. The full list of protected usages is publicly available.

Note that security-sensitive HID devices (such as FIDO HID devices used for stronger authentication) are also blocked in Chrome. See the USB blocklist and HID blocklist files.

Feedback

The Chrome team would love to hear about your thoughts and experiences with the WebHID API.

Tell us about the API design

Is there something about the API that doesn't work as expected? Or are there missing methods or properties that you need to implement your idea?

File a spec issue on the WebHID API GitHub repo or add your thoughts to an existing issue.

Report a problem with the implementation

Did you find a bug with Chrome's implementation? Or is the implementation different from the spec?

Check out How to file WebHID bugs. Be sure to include as much detail as you can, provide simple instructions for reproducing the bug, and have Components set to Blink>HID. Glitch works great for sharing quick and easy repros.

Show support

Are you planning to use the WebHID API? Your public support helps the Chrome team prioritize features and shows other browser vendors how critical it is to support them.

Send a tweet to @ChromiumDev using the hashtag #WebHID and let us know where and how you're using it.

Helpful links

• Specification
• Tracking bug
• ChromeStatus.com entry
• Blink Component: Blink>HID

Acknowledgements

Thanks to Matt Reynolds and Joe Medley for their reviews of this article. Red and blue Nintendo Switch photo by Sara Kurfeß, and black and silver laptop computer photo by Athul Cyriac Ajay on Unsplash.